[30.02] A Lightweight Universe?

N. Bahcall (Princeton University)

What is the mass-density of the universe? and how is the
mass distributed? We show that several independent methods
utilizing clusters of galaxies - including cluster dynamics
and the mass-to-light ratio, baryon fraction, and cluster
evolution - all indicate the same consistent result: the
mass-density of the universe is low, about 0.2-0.3 of the
critical density, and mass approximately traces light on
large scales. When combined with the SNIa and the microwave
background results, the data suggest a universe that is
lightweight, accelerating, and flat.

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